Determining the Best Altitude of NAM and SAM Indices for Assessing Polar Vortex Influence on the Ionosphere-Thermosphere System

The Ionosphere-Thermosphere (I-T) is in the Space-Atmosphere Interaction Region (SAIR), where variability is driven both by impulsive solar events and by waves propagating up from the lower atmosphere. It is well-known that the wintertime polar vortex plays a role in the generation and modulation of lower-atmosphere waves that reach the I-T, and the extent to which the waves couple to the upper atmosphere depends on vortex strength. Most previous studies quantify Arctic vortex strength by calculating the Northern Annular Mode (NAM) index at 10 hPa, a pressure level near 30 km in the middle stratosphere. However, it is also known that polar vortex strength varies with height. This work calculates the NAM, and its southern hemisphere counterpart the SAM, indices at a variety of stratospheric and mesospheric levels to determine the optimum altitude to determine I-T variability. We look at case studies in both hemispheres during the geomagnetically quiet year of 2019, as a proof-of-concept. We then correlate the NAM and SAM indices at a variety of altitudes to variations in thermospheric composition and ionospheric electron density. Finally, we report the best altitude to compute vortex strength and hemispheric asymmetries and sensitivities therein, for studies of vertical coupling between the middle (stratosphere and mesosphere) and upper (thermosphere and ionosphere) atmosphere. This work has ramifications for the predictability of the thermosphere and ionosphere given that the strength of the polar vortex may be predicted up to 2 weeks in advance.